In general, communication is realized through a parallel circuit known as a backplane for each module in a PLC (Programmable Logic Controller) used in an FA (Factory Automation). That is, a backplane is used to communicate from one module to another module, where a module requesting a communication must obtain a control right over the backplane. When several modules try to access to a backplane with no control right, a signal confliction is generated to disable a normal access.
In general, in a communication between a master module requesting a communication and a slave module responding to the request of the master module, a control right over a common resource issues a token according to a predetermined rule, and although access methods are same when a predetermined master possesses a token (i.e., having a control right), it is difficult to realize a satisfactory performance in real time control, because a time for exchanging tokens and a time for processing the tokens are required according to a predetermined rule for determining a token ownership.
FIG. 1 is a schematic view illustrating an exemplary method for obtaining a token-based control right according to a conventional multi-master system.
Referring to FIG. 1, a plurality of masters (100˜400) may possess a control right called a token in a round robin method. The master possessing the token has an authority to access to a backplane, and can access to a slave module through the backplane. That is, when there is a need for a master to control a slave, the master checks if it has a token, and accesses to the slave through a backplane if it has the token, and when the master fails to have a token, and the master cannot access to the slave but has to wait. The master can access to the slave once it has a token, and may transfer the token to other masters according to a predetermined algorithm.
Although FIG. 1 has illustrated a round-robin method, it is possible to receive and transfer the token in other various methods according to a predetermined rule by a performer. However, this method has a limit in performance in real time, because it requires a token control algorithm and token receipt/transmission process time.
FIG. 2 is a schematic view illustrating an exemplary method for obtaining a control right according to a time slot in a conventional multi-master system, where the system is available with n number of masters.
Referring to FIG. 2, a continuous time is divided by time slot using the number of masters or more than the number of masters, where the master can access to a backplane only within a time allocated thereto.
When there is a need to control a slave, a master operates in a manner such that the master first checks if it is a time slot allocated to itself and starts to access the backplane, and the master waits during time slot not allocated to itself, or re-tries after a predetermined period of time.
This method has an advantageous effect in that a processing time can be reduced because there is no direct exchange of tokens but there is a possibility of collision because accesses may be simultaneously tried by time measurement error of each master. That is, a plurality of master systems is driven by respective clocks, and when there is generated a time error on the clocks, there is a possibility of collision due to clock error on same time.